Definitions
Throughout what follows, reference will be made to tubes of the "in-line" type, in which three electron beams are generated in one horizontal plane. However, what follows also applies correspondingly to "delta-gun" tubes.
Because of tolerances, discrepancies occur in the manufacture of color picture tubes between the actual beam position and the reference position. These are static errors (which are present regardless of the properties of a deflector), and dynamic errors (which are caused by the deflector). In what follows, only the static errors are of interest; these are: targeting (color purity), vertical raster position, convergence, and twist. These errors are explained below. They are measured at various points of a screen 10 as depicted schematically in FIG. 7. Convergence and vertical raster position are measured at the center of the screen, as illustrated by a circle labeled KV in FIG. 7. This circle represents, for example, the field of view of a microscope. Measurement points for targeting measurements are labeled LL and LR, while measurement points for the twist measurements carry the designations TL and TR.
FIG. 8 illustrates a raster pattern as visible inside the circle KV. The three electron beams of the tube generate three cross-shaped bar patterns, labeled R, G, and B. For convergence, the cross-shaped bar patterns must essentially coincide. In addition, their horizontal bars should essentially coincide with the horizontal center line H of the tube. In the example of FIG. 8, the three horizontal bars each deviate from the horizontal center line H by values YR, YG, and YB, respectively. A corresponding depiction would consist in indicating the deviation of the green horizontal bar from the horizontal center line, and deviations of the red and blue horizontal bars from the green horizontal bar. The red and blue vertical bars are located at distances XRGK and XBGK, respectively, from the vertical green bar. All deviations can typically be up to several millimeters.
FIGS. 9a and 9b illustrate what is visible inside the circles LL and LR, respectively. In this case the resolution is much finer than in the measurement illustrated by FIG. 8. Specifically, what is being observed is not macroscopic pattern bars, but targeting spots 11 on phosphor stripes 12. At the measurement point LL, the center MSL of the luminous spot 11 is offset 40 .mu.m to the left compared with the center MDL of the phosphor stripe 12. At measurement point LR, however, the corresponding centers MSR and MDR coincide. To adjust the targeting, the electron beams are displaced so that all the luminous spots move 20 .mu.m to the right, so that the luminous spots at measurement point LL are displaced 20 .mu.m to the left with respect to the center of the phosphor stripe, while on the right a corresponding displacement to the right is produced. To produce this 20 .mu.m displacement in targeting, the electron beams must be displaced a few millimeters by means of a static magnetic field in the tube neck.
The aforesaid targeting example clearly shows that a distinction must be made between "beam deviation" and "beam displacement." In what follows, "beam deviation" will be understood to mean deviation from a reference position. "Beam displacement" is understood as the distance by which an electron beam must be displaced on the screen 10, by means of a magnetic field generated in the tube neck, in order to achieve a desired position. This does not immediately need to be the reference position, but may be an intermediate position.
FIGS. 10a to 10c serve to illustrate the twist error mentioned earlier. FIG. 10a depicts what is visible at measurement points TL and TR. The resolution corresponds to that of FIG. 8, and is therefore such that pattern lines R, G, and B can be observed. It is evident that on the left, line R is located above line G, but on the right is below this line, and that the spacing on the left is greater than on the right. Line B is located symmetrically with respect to line R. FIGS. 10b and 10c illustrate that this error is composed of a crossover error (FIG. 10b) and of the twist error itself (FIG. 10c). The individual influences of the two errors can be determined by the two measurements at the points TL and TR.
To conclude the definition of terms, it should be mentioned that "calibration tubes" and "production tubes" will be discussed in what follows. A calibration tube is understood to be any tube which is used to test the sensitivity of a magnetizing device for the adjustment of a magnetizing unit. A production tube, on the other hand, is a tube of the same type on which the errors explained above are measured, and in which a magnet ring is then magnetized by means of a magnetic field that is determined on the basis of the calibration data and the measured deviations. For highly precise adjustments, each individual tube can be used first as a calibration tube and then as a production tube, in each case as explained above. As a rule, however, a magnetizing device is calibrated with the aid of only one tube, and the values obtained with that tube are then applied to many production tubes.